The currently most favored hypothesis advocates a pivotal role for the amyloid precursor protein in the molecular etiology of Alzheimer's disease. Clinical mutations in autosomal dominant AD were shown to increase relative concentrations of the 42 amino acids amyloid β (Aβ) peptide (Suzuki et al., 1994; Scheuner et al., 1996; Citron et al., 1997), a proteolysis product of APP, that shows increased propensity to aggregate (Burddick et al., 1992) and deposit in amyloid plaques in AD brains (Iwatsubo et al., 1994). Also, different missense mutations in the APP gene (APP) cause autosomal dominant early-onset familial AD (Goate et al., 1991) (Alzheimer Disease & Frontotemporal Dementia Mutation Database). Standard molecular diagnostic screening of APP is currently limited to exons 16 and 17—coding in part for the Aβ peptide—and their flanking splice sites. However, it is yet not excluded that genetic variation influencing transcriptional activity of APP might also contribute to disease risk. Aβ peptide production depends largely on the amount of APP substrate and, therefore, it is conceivable that regulation of APP transcription might indeed play an important role in AD susceptibility. In fact, several studies have identified higher levels of APP mRNA in AD brains (for a review, see Theuns and Van Broeckhoven, 2000), and increased expression of APP has been correlated with Aβ deposition in brain in instances such as severe head injury (Gentleman et al., 1993). Perhaps the most convincing evidence came from the observation that APP triplication in Down syndrome (DS) patients leads to an overexpression of APP (Rumble et al., 1989) and deposition of Aβ peptide in neuritic amyloid plaques (Wisniewski et al., 1985), resulting in a 50 year earlier onset of AD symptoms.
APP is expressed in a variety of tissues, with the highest expression levels in neuronal cells of the central nervous system (CNS), and can be induced by a variety of agents such as growth hormones and cytokines as well as stress conditions. Up-regulation of APP transcriptional activity (Siman et al., 1989; Sola et al., 1993) corroborated with the mRNA expression studies (Wirak et al., 1991; Lahiri and Nall, 1995), suggesting a major role for the APP promoter activity in APP expression. The proximal promoter region of APP is devoid of a functional TATA box, shows a high GC content and transcription initiation is regulated by a strong initiator element (Inr) surrounding the major transcription start site (TSS)+1 (Salbaum et al., 1988; La Fauci et al., 1989; Yoshikai et al., 1990; Quitschke et al., 1996). Further, APP promoter activation is mainly governed by two GC-rich elements, the −93/−82 fragment (APBβ) and the −65/−41 fragment (APBα) (Pollwein et al., 1992). Transcriptional activation of APP can also be mediated by heat-shock factor-1 (HSF-1) binding to the heat shock element (HSE) at position −317 following induction by numerous stress factors (Dewji and Do, 1996). Another transcriptional activator was mapped to −350/−366 harboring an AP-1 binding site and flanking the GC box (Querfurth et al., 1999). Of further interest is that members of the NFκβ/Rel family can specifically recognize two identical sequences at −2250/−2241 and −1837/−1822, in the distal promoter region of APP, referred to as APPκβ sites (Grilli et al., 1995). Both the expression patterns and the proximal promoter region of APP are highly conserved between mammalian species (≧80%) (Yamada et al., 1989; Izumi et al., 1992; Chernak, 1993; Song and Lahiri, 1998).
Linkage and association studies support that genetic variability at the APP locus might contribute to increased risk for late-onset AD (Pericak-Vance et al., 1991; Kehoe et al., 1999; Wavrant-De Vrieze et al., 1999; Olson et al., 2002; Meyers et al., 2002; Blaker et al., 2003), in the absence of coding mutations (Lidell et al., 1995). These genetic data further suggested that increased susceptibility might result from genetic mutation in the 5′ regulatory region of APP. However, early on screenings of the APP promoter in sporadic and familial early- and late-onset AD patients, did not reveal AD-specific mutations (Lidell et al., 1995; Fidani et al., 1992; Rooke et al., 1992; Rogaev et al., 1993). More recent studies detected a mutation of G to C at position +37 (hereinafter +37G>C) polymorphism in APP exon 1 while sequencing the −573/+125 fragment of the APP promoter in 20 individuals (Athan et al., 2002). The +37 C-allele was overrepresented in patients with late-onset AD lacking apolipoprotein E (APOE) ε4 alleles (17.2%) compared to elderly control individuals (10%) (OR 2.08, CI 1.26-3.45, adjusted for age, gender and education). Subsequent sequencing of the −308/+124 fragment in 173 patients with late-onset AD and 840 control individuals revealed one more rare variant in control individuals, −9G>C (0.7%), but absent in AD patients. However, both variants, −9G>C and +37G>C, showed no allelic differences in promoter activity when tested in U-87 glioma cells using a reporter gene assay.
The APP locus is known to be complex with several other active sites in the 5′ regulatory region apart from the core promoter. In fact, functional elements that control activity of the human APP promoter are located in three distal regions −2257/−2234, −2250/−2241 and −1837/−1822. Constructions in which the distal region −2435/−2165 has been removed showed an increase in promoter activity (Lahiri et al., 1999). However, this large deletion is not occurring in Alzheimer patients, and no Alzheimer disease-related mutations could be found in this region.